This invention relates to muscarinic receptor ligands with agonist activity. More particularly, this invention relates to compounds based on the tetrahydropyridyl moiety that have unusually high affinity for muscarinic receptors, and exhibit agonist activity useful in the treatment of neurological and other disorders, in which stimulating cholinergic activity is desirable.
Recent advances have been made in the understanding of the cholinergic nervous system and the receptors therein. Cholinergic receptors are proteins embedded in the cell membrane that respond to the chemical acetylcholine. Cholinergic receptors are subdivided into the nicotinic and muscarinic receptor families, and muscarinic receptors represent a family of five subtypes.
Muscarinic receptors mediate a variety of physiological responses to the neurotransmitter acetylcholine in the central and peripheral nervous systems. M1 muscarinic receptors play a role in learning and memory function in the brain and regulate gastric acid secretion in the stomach. M2 receptors regulate acetylcholine release in the central nervous system and control cardiac muscle contraction. Acetylcholine stimulates smooth muscle contraction in a variety of tissues and promotes secretion from exocrine glands. These effects are mediated by M3 receptors. Though less well characterized pharmacologically, M4 receptors appear to play a role in the perception of pain, and M5 receptors may regulate dopaminergic activity in the brain.
It has been suggested that compounds capable of mimicking the action of acetylcholine at these receptors would be useful in treating pathological conditions involving imbalances in these cholinergic pathways. Despite the wealth of knowledge about muscarinic receptor subtypes, relatively few selective ligands are available to characterize muscarinic receptor subtypes. Consequently, the tendency for ligands to bind indiscriminately to muscarinic receptor subtypes has made difficult the development of drugs that are muscarinic receptor subtype selective.
In view of the foregoing, it would be desirable to provide such compounds, particularly so side effects are minimized during treatment of the conditions noted above. It is an object of the present invention to provide compounds having muscarinic receptor affinity and activity. It is another object of the present invention to provide compounds having improved muscarinic receptor selectivity profiles. It is another object of the present invention to provide pharmaceutical composition comprising compounds of the present invention, as active ingredients.
According to one aspect of the present invention, there is provided a compound of Formula I: 
wherein R is a linkage independently selected from (CH2)12 or (CH2)4O3; and acid addition salts, solvates and hydrates thereof.
According to another aspect of the present invention there is provided a pharmaceutical composition comprising compounds of Formula (I) and a pharmaceutically acceptable carrier.
The invention relates to bis-alkyloxy-1,2,5-thiadiazole derivatives of 1,2,5,6-tetrahydropyridine that bind to and activate muscarinic receptors. The compounds incorporate two functional muscarinic agonists into the same molecule with an alkyloxy linkage. More particularly, the present invention is directed to compounds of Formula (I): 
wherein R is a linkage independently selected from (CH2)12 or (CH2)4O3, and acid addition salts, solvates and hydrates thereof.
The compounds of Formula (I), 2, 2xe2x80x2-bis-{[3-(1-methyl-1,2,5,6-tetrahydropyrid-3-yl)-1,2,5-thiadiazol-4-yloxy]ethyloxy}-diethylether and 1, 12-bis-[3-(1-methyl-1,2,5,6-tetrahydropyrid-3-yl)-1,2,5-thiadiazol-4-yloxy]-dodecane, exhibit very high affinity for muscarinic receptors as compared to the parent compound xanomeline. In addition, the compounds appear to interact with multiple M2 receptors expressed in A9 L cells. It is believed that compounds of Formula (I) may act as agonists at muscarinic receptors coupled to the inhibition of adenylyl cyclase activity.
It was heretofore believed that as the length of the alkoxy chain increases agonist activity decreases. As reported in the Journal of Medicinal Chemistry, 1993, Vol. 36, No. 7, pages 843-844, increasing the length of the 3-alkyl chain on the 1, 2, 4-oxadiazole ring of 1, 4, 5, 6-tetrahydropyrimidine dramatically decreased activity in the phosphoinositide metabolism assay. Again these data are consistent with similar observations in 1,24-oxadiazole derivatives of 1,2,5,6-tetrahydro-1-methylpyridine and quinuclidine where increasing the length of the 3-alkyl substituent led to compounds with higher affinity yet lower agonist activity. As shown in Tables 1 and 2, it has been surprisingly found that compounds of Formula I with increasing alkoxy chains displayed M1 agonist efficacy comparable to xanomeline, yet with higher potency and higher affinity for M1 receptors.
The receptor binding properties and agonist activity of bis-thiadiazole derivatives, (Formula (II)), at M1 muscarinic receptors expressed in A9 L cells is provided below in Table 2. PI metabolism represents the percentage stimulation above basal levels at 100 xcexcM expressed relative to the carbachol response (100%). Full dose-response curves were obtained for a few compounds. The data represents the mean (xc2x1s.e.m.) from two to five assays for each compound. 
wherein R is a linkage independently selected from (CH2)2, (CH2)3, (CH2)4, (CH2)5, (CH2)6, (CH2)7, (CH2)8, (CH2)9, (CH2)10, (CH2)12 and (CH2)4O3.
The compounds of Formula (I) are preferably isolated in substantially pure form.
The binding profiles of the compounds of Formula (I) indicate their utility as pharmaceuticals useful for the treatment of various conditions in which the use of a muscarinic receptor ligand is indicated. More particularly, the compounds of Formula (I) have been found to mimic acetylcholine function via an action at muscarinic receptors and are therefore of potential use in the treatment of pain, Alzheimer""s disease and other disorders involving cholinergic deficits. Furthermore, it has been found that the inclusion of heteroatoms in the alkyl chain. Seems to improve the water solubility of the compounds. In addition, agonist activity is enhanced relative to the straight chain derivatives.
The present invention also provides pharmaceutical compositions, which comprise compounds of Formula (I) or pharmaceutically acceptable salts thereof, and pharmaceutically acceptable carriers. The pharmaceutical composition may be in the form of patches, tablets, capsules, powders, granules, lozenges, suppositories, reconstitutable powders or liquid preparations such as oral or sterile parenteral solutions or suspensions. The pharmaceutical composition includes compounds of Formula (I) of a pharmaceutically acceptable level of purity excluding normal pharmaceutical additives such as dilutents and carriers, and including no material considered toxic at normal dosage levels. A pharmaceutically acceptable level of purity will generally be at least 90% excluding normal pharmaceutical additives, preferably 95%, more preferably 97% and still more preferably 99%.
Sauerbeg et al., Journal Medicinal Chemistry, 1992, Vol. 35, page 2274, reported the synthesis and SAR of potent ligands for M1 receptors based on the 1,2,5-thiadiazolyl-tetrahydropyridine moieties. In accordance with the present invention, it was found that if two 1,2,5-thiadiazolyltetrahydropyridine moieties are tethered by spacers of varied length and rigidity, in a single structure, the binding affinity of the resultant bis ligands is enhanced. By varying the length of the alkyl chain and also replacing some of the carbons with heteroatoms like N, O or S, structure activity relationships could be established. The two ligands in the same molecule may either bind in the pockets of two proximal receptors or in two pockets of the same receptor molecule.
The compounds of Formula (I) can be prepared using processes well known in the art.